rxkad: Remove redundant checkCksum initialisation

checkCksum is assigned to by both sides of the if() expression, so
there's no need to initialise it to 0.

Caught by clang-analyzer

Change-Id: Id0e286943e11b6948231577085cdb0ce96bb0b97
Reviewed-on: http://gerrit.openafs.org/9183
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>